The present invention relates to an optical pickup device for use in optical disk devices which optically record information in and/or reproduce information from a data recording medium such as an optical disk. Specifically, the present invention relates to an optical pickup device which enables accurate recording and reproducing operations using an optical disk having a plurality of recording and reproducing layers.
Since optical disks are capable of recording large quantities of information signals at high density, they are being increasingly used in recent years in fields such as audio, video, computers, etc. Recent innovations include recording media which aim to increase recording capacity by recording signals on a plurality of recording layers, and optical systems which aim to reproduce recorded signals at high speed by simultaneously reading signals from a plurality of tracks using a plurality of light beams.
In the foregoing recording media provided with a plurality of recording layers, if the respective recording and reproducing surfaces are too close together, when the light beam is accessing a given recording and reproducing surface, light reflected from that recording and reproducing surface is influenced by light reflected from adjacent recording and reproducing surfaces. In this case, a focus error signal, for focusing adjustment of the light beam, is also subject to the foregoing influence, and thus accurate focusing adjustment cannot be performed.
In an attempt to provide an optical system able to resolve the foregoing problem, the present Applicant has previously proposed the optical pickup device shown in FIG. 11 (Japanese Unexamined Patent Publication No. 9-161282/1997 (Tokukaihei 9-161282), published on Jun. 20, 1997).
In the optical pickup device shown in FIG. 11, light projected by a semiconductor laser 1 passes through a holographic element 2, a collimating lens 3, and an objective lens 4, and is converged on an optical disk 5. Light reflected therefrom passes through the objective lens 4 and the collimating lens 3, and is directed to the holographic element 2.
As shown in FIG. 12(b), the holographic element 2 is divided into three divisions 2a, 2b, and 2c by a dividing line 2g, running in a y direction corresponding to the radial direction of the optical disk 5, and a dividing line 2h, running from the center of the dividing line 2g in an x direction perpendicular to the radial direction of the optical disk 5, i.e., a direction corresponding to the track direction of the optical disk 5.
As shown in FIG. 12(a), a photoreceptor element 7 includes four rectangular photoreceptive domains 7a, 7b, 7c, and 7d arranged along the x direction corresponding to the track direction of the optical disk 5. The central photoreceptive domains 7a and 7b (photoreceptive domains for focusing) are divided from one another by a dividing line 7y running in the y direction corresponding to the radial direction of the optical disk 5, and auxiliary photoreceptive domains 7e and 7f are provided on the outer sides of the photoreceptive domains 7a and 7b. 
The foregoing photoreceptive domains are arranged such that when the light beam is focused on a recording surface of the optical disk 5, reflected light diffracted by the division 2a of the holographic element 2 forms a beam spot P1 on the dividing line 7y, and reflected light diffracted by the divisions 2b and 2c forms beam spots P3 and P2 on the photoreceptive domains 7c and 7d, respectively.
Then, if Sa, Sb, Sc, Sd, Se, and Sf are output signals from the photoreceptive domains 7a, 7b, 7c, 7d, 7e, and 7f, respectively, then a focusing error signal FES is calculated as (Sa+Sf)xe2x88x92(Sb+Se). By this means, an FES curve can be corrected so as to be optimum for a recording medium with a plurality of recording layers.
The following will explain in detail, with reference to FIGS. 13(a) through 13(e), only the photoreceptive domains 7a, 7b, 7c, and 7d and the beam spot P1, which relate to FES. In a focused state, as shown in FIG. 13(a), the beam spot P1, which is reflected light for focusing, is focused on the dividing line 7y. As the optical disk 5 gets farther away, as shown in FIGS. 13(b) and 13(c), the beam spot P1 first spreads into the photoreceptive domain 7b, and is finally incident on the photoreceptive domain 7f as well; as the optical disk 5 gets closer, as shown in FIGS. 13(d) and 13(e), the beam spot P1 first spreads into the photoreceptive domain 7a, and is finally incident on the photoreceptive domain 7e as well.
In FIG. 14, a curve of the focusing error signal FES=(Sa+Sf)xe2x88x92(Sb+Se) is shown as a solid line. Here, outside the pull-in range between xe2x88x92d1 and +d1 where the curve converges with zero, the curve can be brought back to 0 more steeply than the focusing error signal FES when the auxiliary photoreceptive domains 7e and 7f are not provided (=Saxe2x88x92Sb), shown as a broken line, which returns to 0 more gradually. In this case, when reproducing, for example, a two-layer optical disk 5 in which the distance between the layers is d2, the FES curve will be as shown in FIG. 15, giving two independent FES curves (for the two layers) having a sufficiently small FES offset, and thus enabling normal focus servo to be performed.
However, when assembling the optical pickup, there is naturally some assembly error. If the optical pickup is ideally assembled, it is possible, as above, to reduce offset of the focusing error signal by means of light reflected from adjacent recording and reproducing layers, but in the event of assembly error, this changes the shape of the light reflected to the photoreceptor element when focusing operations are performed, which changes the focus error signal correction quantity and makes it impossible to obtain a good FES curve when reproducing an optical disk having a plurality of recording and reproducing layers.
Accordingly, in order to resolve the foregoing difficulties, the present Applicant proposed an optical pickup which, by optimizing the shape of the auxiliary photoreceptive domains, enables accurate recording and reproducing operations on an optical disk having a plurality of recording and reproducing layers, even if assembly error arises during assembly of the optical pickup (Japanese Unexamined Patent Publication No. 10-222867/1998 (Tokukaihei 10-222867), published on Aug. 21, 1998).
The optical pickup disclosed in Japanese Unexamined Patent Publication No. 10-222867/1998 has the same structure as the optical pickup discussed above, but differs in that the shape of the auxiliary photoreceptive domains is optimized by setting their width in the x direction.
The following will explain disturbance of the FES curve which arises in the conventional optical pickup discussed above (Japanese Unexamined Patent Publication No. 9-161282/1997) due to error in assembly.
In particular, FIG. 16 shows an FES curve when the holographic element 2 shown in FIGS. 11 and 12(b) is misadjusted with an offset in the +x direction with respect to an optical axis determined by the semiconductor laser 1 and the collimating lens 3. In this case, in a greatly defocused state, a large peak is produced when defocusing is in the far direction (at around +d2). In this state, reproducing, for example, a two-layer optical disk in which the distance between the layers is d2 gives rise to an FES offset of xcex94d1, and a correctly focused state cannot be obtained.
Since FES offset due to incorrect positioning of the holographic element, tolerance of the various members, laser wavelength aberrance, etc. is generally adjusted to 0 by rotation adjustment of the holographic element 2, it does not create a problem in single-layer optical disks, but in a greatly defocused state, the shape of the light beam spot differs from the ideal shape, causing this kind of disturbance of the FES.
The foregoing change in the shape of the light beam spot will now be explained with reference to FIGS. 17(a) through 17(e). If the dividing line 2g of the holographic element 2 is offset in the +x direction, greatly defocused reflected light, as shown in FIGS. 17(c) and 17(e), exceeds the dividing line 7y and extends into the photoreceptive domains 7a and 7b, respectively. When defocusing is in the far direction (FIG. 17(c)), in particular, the reflected light greatly exceeds the dividing line 7y and is incident on the photoreceptive domain 7a, on which it would not be incident in the absence of assembly error. If xcex94Sa is the amount of increase in the signal Sa from the photoreceptive domain 7a, then the focus error signal FES=(Sa+Sf)xe2x88x92(Sb+Se) is subject to a disturbance calculated as follows:                     FES        =                              (                          Sa              +              Sf                        )                    -                      (                          Sb              +              Se                        )                                                  =                  Δ          ⁢                      xe2x80x83                    ⁢          Sa                                        =                  Δ          ⁢                      xe2x80x83                    ⁢          d1                    
The following will explain the case of offset of the dividing line 2g of the holographic element 2 in the xe2x88x92x direction. As shown in FIGS. 18(a) through 18(e), greatly defocused reflected light recedes from the dividing line 7y. In the absence of incorrect positioning of the dividing line 2g , the reflected light is incident on the entirety of the photoreceptive domain 7b (as shown in FIG. 13(c)), but in the event of incorrect positioning, as shown in FIG. 18(c), the amount of reflected light incident on the photoreceptive domain 7b is reduced. If xcex94Sb is the amount of decrease in the signal Sb from the photoreceptive domain 7b, then the focus error signal FES=(Sa+Sf)xe2x88x92(Sb+Se) is subject to a disturbance calculated as follows:                     FES        =                              (                          Sa              +              Sf                        )                    -                      (                          Sb              +              Se                        )                                                  =                  Δ          ⁢                      xe2x80x83                    ⁢          Sb                                        =                  Δ          ⁢                      xe2x80x83                    ⁢                      d1            xe2x80x2                              
In order to resolve this problem, Japanese Unexamined Patent Publication No. 10-222867/1998 uses a method which reduces the surface area of each auxiliary photoreceptive domain. Reducing the widths of the auxiliary photoreceptive domains 7e and 7f yields the FES curve shown by a solid line in FIG. 19. Further, in the event of assembly error, an FES curve is as shown by a solid line in FIG. 20. As is evident from a comparison between FIGS. 20 and 16, reducing the widths of the auxiliary photoreceptive domains 7e and 7f can reduce the peak xcex94d1 when defocusing is in the far direction, which arises due to incorrect positioning of the holographic element 2.
However, when the optical pickup is ideally assembled, since the FES correction quantity in this case is small, the rise in the FES curve is more gradual than in the case of the FES (shown by a dot-and-dash line) obtained with wider auxiliary photoreceptive domains (FIG. 14). Thus, at a defocusing position of +d2, for example, a slight focusing offset of xcex94d2 arises.
In this way, in the foregoing example, due to the fact that the auxiliary photoreceptive domains for detecting a defocused state were positioned symmetrically with respect to a dividing line formed in the main photoreceptive domain, and on the outer sides of the main photoreceptive domain, a good FES curve could only be obtained either when there was assembly error or when there was no assembly error, but not in both cases.
It is an object of the present invention to provide an optical pickup device which contributes to improvement of an FES curve of a multi-layer optical disk; in particular, an optical pickup device in which there is no interference among FES curves of the various layers of a multi-layer optical disk having a small distance between recording and reproducing layers, which is able to produce an FES with little disturbance even in cases of assembly error, and which can also be applied to an optical system with a plurality of beams with a small inter-beam interval.
In order to attain the foregoing object, an optical pickup device according to the present invention comprises a light source, an optical system which converges light projected by the light source onto a recording medium and directs reflected light from the recording medium to a photoreceptor element, and a photoreceptor element which detects the reflected light;
the photoreceptor element including at least two main photoreceptive domains divided from one another by a dividing line, which receive reflected light corresponding to a focusing error of the light projected onto the recording medium, and an auxiliary photoreceptive domain which detects reflected light which exceeds the main photoreceptive domains in a defocused state;
in which the auxiliary photoreceptive domain is provided adjacent to an end of said main photoreceptive domains in the direction of the dividing line.
With the foregoing structure, the FES curve in the area outside a pull-in range can be improved, there is no interference among FES curves (no offset) of the various layers of a multi-layer optical disk having a small distance between recording and reproducing layers, and an FES with little disturbance can be produced even in the event of assembly error. Further, the structure according to the present invention can also be applied to an optical system with a plurality of beams with a small inter-beam interval.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.